All thymic progenitor activity in blood is found within the rare Flk2⁺CD27⁺ subset. (A) Blood was isolated from 20 mice, lineage depleted, and stained to identify lineage⁻Flk2⁺CD27⁺ cells and MPPs and CLPs within this population. The lower left plot shows the enriched lineage⁻Flk2⁺CD27⁺ population. IL-7R⁺ and IL-7R⁻ cells are present within this population, and the c-Kit versus Sca-1 profiles of both groups are displayed. (B) All lineage⁻Flk2⁺CD27⁺ cells (4500) and all remaining lineage⁻ cells (2.5 × 10⁶) were sorted from the blood of 10 mice and injected into sublethally irradiated congenic recipients as in Figure 2. After 14 days, the contribution of donor thymocytes was determined. All T progenitor activity was within the Flk2⁺CD27⁺ blood subset, and thymocytes had reached the CD4⁺CD8⁺ DP stage of development. Plots are representative of 3 mice each in each of 2 experiments.